1. Field of the Invention
The present invention relates to an apparatus and process for improving the visibility of objects within a visual field and in particular to an apparatus and process for use in an automotive vehicle under adverse optical conditions.
2. Description of the Prior Art
Currently, the most common method of improving the visibility of objects within a forward visual field of an automotive vehicle during night driving is to increase the intensity of light cast by the headlights. However, the intensified headlights tend to dazzle the drivers of vehicles in the opposite lane.
Japanese published unexamined patent application No. 49-72830 discloses a system for reducing the transmission of intensified headlight cast by an automotive vehicle in the opposite lane through a vehicular windshield by means of an optical filter with adjustable transmissivity e.g., a liquid crystal panel and an electronic circuit for controlling the transmissivity of the optical filter. However, since this system darkens the forward visual field of the vehicle on which the headlight is incident, at the moment the optical filter transmissivity is reduced, the driver of the filtering vehicle may fail to recognize pedestrians, etc. crossing or standing in the road within the forward visual field.
Japanese published unexamined patent application No. 52-101526 also discloses a system for shutting out the headlight cast by a vehicle in the opposite lane. However, this system is less effective in cases where the vehicle in the opposite lane lacks the same polarizing filters or plates as are used on one's own vehicle.
Generally, it is known that, when an object is illuminated stroboscopically at a low frequency, an observer will see a flickering image of the illuminant, but if the on-off frequency of the illuminant is increased to a certain value, the observer will have a steady image of the illuminant. With reference to human eyesight, this effect is quanitized in Talbot-Plateau's law: ##EQU1## wherein L.sub.m is steady luminance with time and L(t) is varying luminance with time. In addition, the critical fusion frequency (hereinafter refer to as CFF) at which an observer will perceive a steady image of the illuminant can be derived from Ferry-Porter's law as follows: EQU CFF=a log L.sub.m +b
wherein a and b are constants.